A gel-based speaker has demonstrated the viability of devices powered by electrical charges from ions rather than electrons.

The transparent device consists of a thin sheet of rubber sandwiched between two layers of a saltwater gel that can produce sounds that span the entire audible spectrum, 20 hertz to 20 kilohertz, when a high-voltage signal runs across the surfaces and through the layers, forcing the rubber to rapidly contract and vibrate.

But the speaker is not an electronic device as it relies on electrical charges carried by ions, rather than electrons. Published in journal Science today, the device represents the first demonstration that ionic conductors can be put to meaningful use in fast-moving, high-voltage devices.

“Ionic conductors could replace certain electronic systems; they even offer several advantages,” said co-lead author Jeong-Yun Sun, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences (SEAS).

Among the promising characteristics of ionic conductors is the fact that they can be stretched to many times their normal area without an increase in resistivity – a problem common in stretchable electronic devices – and they can also be transparent making them well suited for optical applications.

Perhaps more importantly the gels used as electrolytes are biocompatible as signals carried by charged ions are the way the body transmits electrical signals, so it should be relatively easy to incorporate ionic devices – such as artificial muscles or skin – into biological systems.

“The big vision is soft machines,” said co-lead author Christoph Keplinger, who worked on the project as a postdoctoral fellow at SEAS in the Department of Chemistry and Chemical Biology.

“Engineered ionic systems can achieve a lot of functions that our body has: they can sense, they can conduct a signal, and they can actuate movement. We’re really approaching the type of soft machine that biology has to offer.”

The audio speaker represents a proof of concept for ionic conductors because producing sounds across the entire audible spectrum requires both high voltage to squeeze hard on the rubber layer and high-speed actuation to vibrate quickly.

In the past researchers have found that high voltages can set off electrochemical reactions in ionic materials, producing gases and burning up the materials, and also that ions are also much larger and heavier than electrons so physically moving them through a circuit is typically slow.

By overcoming both problems the system invented at Harvard opens up a host of potential applications including biomedical devices, but also fast-moving robotics and adaptive optics.

“It must seem counterintuitive to many people, that ionic conductors could be used in a system that requires very fast actuation, like our speaker,” said Sun.

“Yet by exploiting the rubber layer as an insulator, we’re able to control the voltage at the interfaces where the gel connects to the electrodes, so we don’t have to worry about unwanted chemical reactions.

“It must seem counterintuitive to many people, that ionic conductors could be used in a system that requires very fast actuation, like our speaker,” said Sun.

“Yet by exploiting the rubber layer as an insulator, we’re able to control the voltage at the interfaces where the gel connects to the electrodes, so we don’t have to worry about unwanted chemical reactions.

“The input signal is an alternating current (AC), and we use the rubber sheet as a capacitor, which blocks the flow of charge carriers through the circuit. As a result, we don’t have to continuously move the ions in one direction, which would be slow; we simply redistribute them, which we can do thousands of times per second.”

The Harvard team chose to make its audio speaker out of simple materials – the electrolyte is a polyacrylamide gel swollen with salt water – but they say an entire class of ionically conductive materials is available for experimentation.

They plan to focus future work on identifying the best combinations of materials for compatibility, long life, and adhesion between the layers.

“We’d like to change people’s attitudes about where ionics can be used,” said Keplinger.

“Our system doesn’t need a lot of power, and you can integrate it anywhere you would need a soft, transparent layer that deforms in response to electrical stimuli; for example, on the screen of a TV, laptop, or smartphone to generate sound or provide localized haptic feedback; and people are even thinking about smart windows.

“You could potentially place this speaker on a window and achieve active noise cancellation, with complete silence inside.”

Sam Liss, director of business development in Harvard’s Office of Technology Development, is working to commercialize the technology by working with companies involved in everything from tablet computing to smartphones, wearable electronics, consumer audio devices, and adaptive optics.

“With wearable computing devices becoming a reality, you could imagine eventually having a pair of glasses that toggles between wide-angle, telephoto, or reading modes based on voice commands or gestures,” suggested Liss.